1. Field of the Invention
The present invention relates to a power supply, an electronic device using the same, and an output short-circuit protection method for the same.
2. Description of the Related Art
FIG. 7 shows the circuit diagram of a conventional power supply. In FIG. 7, a power supply 1 includes a direct-current power supply Vcc, transistors Q1 and Q2, an operational amplifier OP1, a Zener diode ZD, resistors R1, R2, R3, and R4, capacitors C1 and C2, and an output terminal Pout.
The direct-current power supply Vcc is connected to the collector of the transistor Q1, which is a regulator element. The emitter of the transistor Q1 is connected via the resistor Rs to the output terminal Pout. Two input terminals of the operational amplifier OP1 are connected across the resistor Rs, and the output of the operational amplifier OP1 is connected to the base of the transistor Q1. The base of the transistor Q1 is connected to the collector of the transistor Q2. The emitter of the transistor Q2 is coupled to ground via the Zener diode ZD. The cathode of the Zener diode ZD is connected via the resistor R1 to the direct-current power supply Vcc. The resistor R2 is connected between the base and the collector of the transistor Q1. The direct-current power supply Vcc is connected in parallel with the capacitor C1. A capacitor is connected between P and ground and C2 and it is also grounded via the resistors R3 and R4 are connected in parallel, as shown, to capacitor C2. The node between the resistors R3 and R4 is connected to the base of the transistor Q2.
In the power supply 1 having such a construction, the emitter of the transistor Q2 is maintained at a reference voltage Vref by the resistor R1 and the Zener diode ZD. The voltage of the output terminal Pout is detected using the resistors R3 and R4. The transistor Q2 is controlled using the detected voltage. The transistor Q2 controls the transistor Q1 so that the voltage of the output terminal Pout is maintained based on the reference voltage Vref. The capacitors C1 and C2 are smoothing capacitors. The resistor R2 is a starting resistor for the transistor Q1. In addition, a load (not shown) is connected between the output terminal Pout and ground.
An output short-circuit protection operation at the time when short-circuiting occurs at the output side is described. In this application, short-circuiting means not only complete short-circuiting but also a case in which a large amount of current flows through the load because the resistance of the load is decreased below a predetermined resistance. The output of the transistor Q1 is connected in series with the resistor Rs. The voltage drop across the resistor Rs is generated in proportion to the magnitude of the output current and is input into the operational amplifier OP1. When a voltage drop of a predetermined value or above is input into the operational amplifier OP1, the output voltage is decreased. Therefore, a large amount of current flowing through the resistor Rs due to the occurrence of short-circuiting at the output side causes the base voltage of the transistor Q1 to decrease. When the base voltage of the transistor Q1 is decreased, the transistor Q1 is turned off, preventing the output current from flowing. That is, in the power supply 1, the output short-circuit protection operation is realized using the resistor Rs and the operational amplifier OP1.
FIG. 8 shows the circuit diagram of another conventional power supply. Components in FIG. 8 that are equivalent to the corresponding components in FIG. 7 have the same reference numerals and descriptions thereof are omitted.
In FIG. 8, a power supply 2 includes transistors Q3 and Q4, resistors R5, R6, and R7, and capacitor C3 instead of the resistor Rs and the operational amplifier OP1 in the power supply 1 shown in FIG. 7. The direct-current power supply Vcc is connected to ground via the resistor R5 and the capacitor C3 in this order. The node between the resistor R5 and the capacitor C3 is connected to the emitter of the transistor Q3. The output terminal Pout is connected to ground via the resistors R6 and R7 in this order. The node between the resistors R6 and R7 is connected to the base of the transistor Q3. The collector of the transistor Q3 is connected to the base of the transistor Q4. The collector of the transistor Q4 is connected to the base of the transistor Q1, and the emitter of transistor Q4 is grounded.
In a case in which the load is normal, the operation of the power supply 2 having such a construction is similar to that of the power supply 1. The description of the operation of the power supply 2 in this case is therefore omitted.
When short-circuiting occurs at the output side of the power supply 2, an output short-circuit protection operation is described. Since the resistors R6 and R7 are connected between the output terminal Pout and the ground, the output voltage can be found at the node with the output terminal Pout. When short-circuiting occurs at the output side, voltage drop occurs at the node between the resistors R6 and R7, and the transistor Q3 is turned on. When the transistor Q3 is turned on, the current flows via the resistor R5 and the transistor Q3 into the base of the transistor Q4, and the transistor Q4 is turned on. When the transistor Q4 is turned on, the collector voltage of the transistor Q4 is decreased. Accordingly, since the base voltage of the transistor Q1 connected to the collector of the transistor Q4 is also decreased, the transistor Q1 is turned off. This prevents the output current from flowing. That is, in the power supply 2, the output short-circuit protection operation is realized using the transistors Q3 and Q4, the resistors R5, R6, and R7, and the capacitor C3.
When the power supply 2 is first turned on, since the base voltage of the transistor Q3 is substantially 0V, the output short-circuit protection operation would be immediately activated. Therefore, a delay circuit including the resistor R5 and the capacitor C3 for preventing the transistor Q3 from turning on when the power is first turned on is provided. Due to the operation of this delay circuit, a significant amount of time is required for the emitter voltage of the transistor Q3 to increase. Since the transistor Q3 is not turned on during this time, unnecessary output short-circuit protection operation is not caused at initial turn on of the power supply.
In the power supply 1 shown in FIG. 7, since the path through which the output current flows is connected in series with the resistor Rs, even when the power supply is operated normally, electrical power is consumed at the resistor Rs. This decreases the efficiency of the power supply 1.
In the power supply 2 shown in FIG. 8, the delay circuit must be provided so that the output short-circuit protection operation is not activated due to the operation of the transistor Q3 when the power is turned on. Provision of the delay circuit increases the circuit size, which prevents the power supply from being miniaturized and from being less expensive. Furthermore, in the power supply 2, when the transistor Q1 is turned off due to the output short-circuit protection operation, the voltage of the output terminal Pout is 0V. Therefore, even though the short-circuit state at the output side is eliminated, the output short-circuit protection operation remains and is not restored to the normal state. Accordingly, once the output short-circuit protection operation is activated, the power must be turned on again in order to cause the power supply to restore the power supply to the normal state.
Accordingly, objects of the present invention are to solve the foregoing problems. There are provided a power supply which can perform an output short-circuit protection operation using a simple circuit, an electronic device using the same, and an output short-circuit protection method for the same.
To this end, according to a first aspect of the present invention, there is provided a power supply for converting a first direct-current voltage into a second direct-current voltage and outputting the second direct-current voltage. This power supply includes a reference voltage generating circuit for generating a reference voltage, a control circuit for controlling an output voltage in accordance with the reference voltage, and a short-circuit detecting circuit for performing an output short-circuit protection operation by, when short circuiting occurs at the output side, detecting the occurrence of short circuiting and by decreasing the reference voltage so that the output voltage is decreased.
The power supply may further include a regulator element controlled by the control circuit.
The power supply may further include a switching element which is switched on and off by the control circuit.
In the power supply, the second direct-current voltage may be lower than the first direct-current voltage.
In the power supply, the short-circuit detecting circuit may be a diode connected between the output side and the reference voltage generating circuit.
According to a second aspect of the present invention, an output short-circuit protection method for a power supply converts a first direct-current voltage into a second direct-current voltage and outputs the second direct-current voltage. The output short-circuit protection method comprises the steps of generating a reference voltage, stabilizing the output voltage in accordance with the reference voltage, and performing an output short-circuit protection operation by decreasing the reference voltage when short circuiting occurs at the output side so that the output voltage is decreased.
By constructing the power supply in the above-described manner, in the power supply and the output short-circuit protection method of the present invention, the output short circuit protection method can be performed using a simple circuit.
According to the third aspect of the present invention, an electronic device includes a power supply according to the first aspect of the present invention.
The electronic device according to the present invention can be miniaturized and inexpensive while improvement in the operation thereof is achieved.